Thoriated Tungsten Filament as ION Source
An ION source is a device that ionizes neutral atoms or molecules and draws out an ion beam. With more and more popular of ion accelerators, mass spectrometers, electromagnetic isotope separators, ion implanters, ion beam etching devices, ion thrusters, and controlled fusion devices, as indispensable components of these equipment, ion sources have many types and uses.
The generation of ions for an ion source is based fundamentally on three main processes, which are electron impact ionization, photo-induced ionization and surface ionization. In addition to these, some other atomic and molecular processes can be the process leading to ion extraction, for example, charge exchange between ions and atoms, and molecular dissociation into atoms and ions.
Ion sources have always been soundly rooted in the research arena, which has led to continual optimization of the design for each application type and results in an enormous array of ion source types, including 1) electron impact ionization sources; 2) plasma discharge ion sources; 3) RF discharge; 4) electron cyclotron resonance ion sources (ECRIS); 5) laser ion sources; 6) surface ion sources; 7) charge exchange ion sources.
To separate molecules and extract ions, the ion source device needs the assistance of thoriated tungsten filament. The filament is made by adding thorium rare elements on the basis of tungsten powder, using powder metallurgy, and drawing into a specified size finally. Thorium oxide ("thoria"), by itself an insulator, adding to the tungsten with greatly reduce the work function. Put another way, the fermi level of thorium is higher than tungsten's, thus closer to the energy of free electrons in vacuum.
A thoriated tungsten emitter is usually operated at a temperature in the vicinity of 2000°K (1727°C), when a good emission life is realized together with a reasonable emission efficiency. Nowadays, lighting manufacturers, analysis and medical equipment manufacturers, etc. prefer to using thorium tungsten filament as an ion source.
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